Casting is an old and well-known art in which liquefied materials are poured or injected into a mold which has a cavity of a desired shape. The liquefied materials are then allowed to solidify to create a cast article. Metal casting is one type of casting in which molten metals are introduced into a mold cavity at high temperatures and then allowed to solidify as the metals cool. The rate and pattern of cooling can affect the solidification process can directly affect the structure of the final cast article. For example, in some cases portions of a casting may begin to solidify at many different locations, leading to multidirectional solidification patterns within the casting. In other cases, premature cooling or cooling at undesired rates can result in undesirable microstructures within the casting or may clog or otherwise block portions of a mold cavity resulting in an unfinished or partial casting.
Solidification of a molten material within a mold cavity most frequently begins when the molten material first contacts the side walls or other inner surfaces of the mold cavity. Because molten materials are usually much hotter than the mold itself, heat quickly escapes from the molten material into the mold upon first contact. Once the material first contact the mold wall, solidification can spread rapidly through the molten material throughout the cavity. For example, molten metals exhibit an extremely high rate of heat loss and once solidification begins, an entire amount of molten metal within a mold can freeze almost instantaneously. The period of time extending from the first introduction of the molten material until complete solidification is often referred to as the dwell time. As will be appreciated, dwell times can be extremely short for molten materials, especially for molten metals, and sometimes may only last for a few seconds or even less than a second (e.g., milliseconds).
In some cases, it can be helpful to extend the dwell time of a molten material. For example, lengthening the dwell time may ensure adequate time for filling a mold cavity before solidification is complete or may promote the growth of desirable grain patterns in the solidifying material. In addition, a longer dwell time may facilitate additional activities during the casting process. As an example, when casting composite materials, extending the dwell time can allow more flexibility in positioning inserts, preforms, and other composite materials within the casting before solidification is complete.
Past efforts to extend dwell time include a few different approaches. Some have tried to increase dwell time by minimizing the temperature difference between surface of the mold cavity and the molten material being introduced into the mold. Some efforts included heating the mold to a temperature closer to that of the molten material. Other efforts included lowering the temperature of the molten material to a temperature closer to that of the molten material. While these efforts have been somewhat helpful, they have not been practical because increasing the temperature of the mold can consume large amounts of energy and may be inherently limited by the melting point of the mold material.